<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>
    
   
</head>
<body>
<form name="form1" method="post" action="">

<select id="s" >
<option value="1" selected="selected">Hello0</option>
<option value="2" >Hello1</option>
<option value="3" >Hello2</option>
<option value="4" >Hello3</option>
</select>
<input name="" type="button" onClick="document.getElementById('s').value=2" value="hellow">
</form>

</body>